Anti-reflection films are used for various image display devices, such as liquid crystal displays (LCD), plasma display panels (PDP), electro luminecence displays (ELD), and cathode-ray tube displays (CRT). Further, anti-reflection films are also used for lenses of glasses or cameras.
As an anti-reflection film, a multi-layered film, in which transparent metal oxide thin films are laminated, has been ordinarily used. Multiple transparent thin films are used to prevent reflection of lights of various wavelengths. The transparent metal oxide thin film is formed by a chemical vapor deposition (CVD) process or a physical vapor deposition (PVD) process, and particularly by a vacuum vapor deposition process, which is a physical vapor deposition process. The transparent metal oxide thin film has excellent optical characteristics as an anti-reflection film. However, the method of forming a transparent matal oxide thin film by such vapor deposition has insufficient productivity for mass production.
In place of the vapor deposition process, methods have been proposed, in which a coating solution containing inorganic fine particles is applied, to form an anti-reflection film.
JP-B-60-59250 (“JP-B” means an examined Japanese patent publication) discloses an anti-reflection layer comprising micro voids (cavities) and inorganic fine particles. The anti-reflection layer is formed by a coating method. Then, the thus-formed layer is subjected to active gas treatment, whereby the gas escapes from the layer, to form micro voids.
JP-A-59-50401 (“JP-A” means an unexamind published Japanese patent application) discloses an anti-reflection film comprising a support, a high-refractive-index layer, and a low-refractive-index layer, superposed in this order. This publication also discloses an anti-reflection film further comprising a middle-refractive-index layer, superposed between the support and the high-refractive-index layer. The low-refractive-index layer is formed by coating a polymer or inorganic fine particles.
JP-A-2-245702 discloses an anti-reflection film comprising a mixture of two or more kinds of ultra fine particles (e.g. MgF2, SiO2), whose mixing ratio is designed to be different in the direction of film thickness. By changing the refractive index due to the different mixing ratio, the film of this publication attained similar optical characteristics to those of the anti-reflection film disclosed in JP-A-59-50401, which film comprises both high- and low-refractive-index layers. The ultra-fine particles are adhered with SiO2 which is formed by thermal decomposition of ethyl silicate. In the thermal decomposition of ethyl silicate, the ethyl part thereof is burned, to evolve carbon dioxide and water vapor. As illustrated in FIG. 1 of the publication, the above-mentioned carbon dioxide and water vapor escape from the layer, to form voids among the ultra-fine particles.
JP-A-5-13021 discloses changing the voids among ultra-fine particles existing in the anti-reflection film described in the above JP-A-2-245702, with a binder. JP-A-7-48527 discloses an anti-reflection film containing inorganic fine particles of porous silica, and a binder.
JP-A-8-110401 and JP-A-8-179123 disclose a technique that a high-refractive-index layer, having a refractive index of 1.80 or more, is made by incorporating inorganic fine particles having a high refractive index into a plastic, and then the high-refractive-index layer is applied to an anti-reflection film.
High-refractive-index Layer Formation:
A method for making a high-refractive-index layer by application of inorganic fine particles has high productivity and is suitable for mass production.
A transparent high-refractive-index layer is formed by finely dispersing inorganic fine particles, and then forming a high-refractive-index layer while the finely dispersed state is kept. By incorporating a larger amount of inorganic fine particles having a high refractive index into a high-refractive-index layer, the formed high-refractive-index layer comes to have a higher refractive index.
It is very effective to incorporate fine particles of titanium dioxide, which are colorless and have an especially high refractive index, into a high-refractive-index layer.
Meanwhile, a high-refractive-index layer is arranged on a display face of an image display device or an outside surface of a lens. Therefore, for the high-refractive-index layer, high physical strengths (abrasion resistance and the like), and weathering resistance (light resistance, moisture/heat resistance, and the like) are required. Fine particles of titanium dioxide have a photocatalyst function to decompose organic compounds that contact the particles and deteriorate the physical strengths, transparency, and the like, remarkably. Furthermore, the fine particles cause a drop in the refractive index of any high-refractive-index layer. Such a phenomenon arises remarkably, in particular, in high-refractive-index layers containing fine particles of titanium dioxide that keep finely dispersed state.
Low-refractive-index Layer Formation:
A low-refractive-index layer having a very low refractive index can be obtained by forming micro voids among fine particles contained in the layer. Since the low-refractive-index layer is placed on the display face of an image display device or on the outer surface of a lens, the layer needs to have sufficient mechanical strength. Further, since the low-refractive-index layer is placed as mentioned above, the layer must have very few defects on the surface (e.g. pointing defect on the surface), to prevent the deterioration of visibility.
The low-refractive-index layer described in JP-A-2-245702 had voids among piled fine particles, so that the refractive index of the layer was very low. However, there was a problem that the low-refractive-index layer described in the publication substantially consisted of only an inorganic compound, and therefore it was very fragile.
JP-A-11-006902 describes a low-refractive-index-layer in which at least two inorganic fine particles were piled, to form voids among these fine particles, thereby obtaining a low-refractive-index layer having both a very low refractive index and high mechanical strength.
JP-A-9-288201 discloses an anti-reflection layer having a low-refractive-index layer in which, by piling up two or more layers of fine particles comprising a fluorine-containing polymer, voids were made between the fine particles.
On the other hand, when an anti-reflection film is formed by applying, onto a substrate, a low-refractive-index layer as described, for example, in JP-A-2-245702, a problem arises that surface defects (pointing defects) are apt to occur, and consequently the thus-produced anti-reflection film is unsatisfactory.
The construction of a conventional liquid crystal display type image display device is shown in FIG. 12. An ordinary liquid crystal display type image display device is composed of a backlight 211 of an edge light type on the furthest back surface and, in the order from the furthest back surface, a light introductive plate 212 for injecting light from the back light toward the surface, a scattering sheet 213 for uniformly dispersing brightness of the light, and one or plural light-tuning sheet (light tuning film) 214 having a function for condensing the uniformly dispersed light by the scattering sheet to a given direction or alternatively a function for selectively transmitting or reflecting a specific polarized light. Light passing through these films is injected to a liquid crystal cell 217 interposed between a pair of polarizing plates 215 (backside polarizing plate) and 216 (surface polarizing plate). In the figure, 218 denotes a cooled cathode fluorescent tube as light source, and 219 denotes a reflective sheet.
An anti-reflection film is generally arranged on a topmost surface of a display device, wherein the principle of optical interference is used to reduce its reflectivity, in an image display device, such as a CRT, a PDP, or an LCD, in order to prevent a drop in contrast or projection of an image by reflection of outside light. That is, in FIG. 12, an anti-reflection film is deposited on a displaying side of 216.
In the meantime, the display type of LCD can roughly be classified into a birefringence mode and an optical rotation mode. A super twisted nematic liquid crystal display device utilizing the birefringence mode (referred to hereinafter as STN-LCD) employs super twisted nematic liquid crystal possessing a twisted angle exceeding 90° and steep electrooptical characteristics. Therefore, STN-LCD enables display of a large capacity due to multiplex drive. However, STN-LCD has problems such as a slow response (several hundred milliseconds) and difficulty in gradation display, and is inferior in display characteristics to those of a liquid crystal display device using active element (such as TFT-LCD and MIM-LCD).
In TFT-LCD and MIM-LCD, a twisted nematic liquid crystal possessing a twisted angle of 90° and a positive birefringence is used for displaying images. These are a display mode of TN-LCD, which is an optical rotation mode. As this mode obtains a high responsiveness (several ten milliseconds) and a high contrast, this mode is advantageous in many aspects as compared with the birefringence mode. Since TN-LCD changes display colors and display contrast according to a viewing angle of looking at the liquid crystal display device (viewing angle characteristics), it involves a problem that the device is not easy to watch as compared with CRT.
JP-A-4-229828 and JP-A-4-258923 respectively disclose a proposal of providing a phase differential plate (optical compensative sheet) between a liquid crystal cell and a pair of polarizing plate for improving viewing angle characteristics. As the phase differential plate proposed in the aforesaid publications has a phase difference of almost 0 in the vertical direction to the liquid crystal cell, it gives no optical effect on direct front but a phase difference is realized when it is tilted. A phase difference generated in an inclined direction is thereby compensated. A sheet having a negative birefringence so as to compensate a positive birefringence of a nematic liquid crystal and having an inclined optic axis is effective for such optical compensative sheet.
JP-A-6-75115 and EP 0576304A1 respectively disclose an optical compensative sheet having a negative birefringence and an inclined optic axis. This sheet is manufactured by stretching a polymer such as polycarbonate or polyester, and has a main-refractive-index direction inclined to the normal line thereof. As such sheet requires an extremely complicate stretching treatment, therefore, it is extremely difficult to manufacture a uniform optical compensative sheet of a large area stably according to this method.
On the other hand, JP-A-3-9326 and JP-A-3-291601 respectively disclose a method using a liquid crystal polymer. An optical compensative sheet is thereby obtained by applying a liquid crystal polymer onto the surface of an alignment (oriented) layer on a support. However, as the liquid crystal polymer fails to show a satisfactory alignment on the alignment layer, it is impossible to enlarge the viewing angle in all directions. Further, JP-A-5-215921 discloses an optical compensative sheet (birefringent plate) that comprises a support and a liquid crystal polymeric bar-type compound having a positive birefringence. This optical compensative sheet is obtained by applying a solution of the polymeric bar-type compound onto the support and curing the compound under heating. However, the liquid crystal polymer is devoid of birefringence so that it is unable to enlarge the viewing angle in all directions.
In JP-A-8-50206, there is disclosed an optical compensative sheet characterized by a layer of a negative birefringence comprising a compound having a discotic structure unit, wherein an angle between the discotic compound and a support is changed in the direction of the depth of the layer. According to the method described therein, a viewing angle viewed from contrast is extensively enlarged in all directions, and deterioration of images such as yellowing viewed from an incline direction is scarcely observed. With the optical compensative sheet alone, however, a deterioration in display quality based on reflection of outside light as mentioned above, cannot be improved. Thus, further improvement is required.